Because they don't see technical debt as a bad thing. Go back to the original pitch for technical debt by Ward Cunningham. He was working for a finance company and used the debt metaphor to advocate in favor of "borrowing against the future" when implementing new code. The idea was that you could use the latest stuff without reinventing everything. Like mortgage payments, the reinvention could happen over time, in increments, until it's "paid off". The problem is that nowadays, especially in finance, you're dealing with people who are used to over leveraging everything. To them, the idea is to pile on debt until it becomes unmanageable: and then sell. Or go into Chapter 11.

Rewriting an application will usually simply give you a more buggy app with less features and more technical dept. At a high cost.

Smart businesses and developers instead step-by-step refactor and upgrade their existing apps.

Move fast and break things has downsides that get to be paid some years down the road.